1. Field of the Invention
The present invention relates generally to the selective modification and control of a patient""s body temperature. More particularly, the present invention provides methods and apparatus for treating hypothermia or hyperthermia by inserting a catheter into a blood vessel of the patient and selectively transferring heat to or from blood flowing through the vessel.
2. Description of the Background Art
Under ordinary circumstances the thermoregulatory system of the human body maintains a near constant temperature of about 37xc2x0 C. (98.6xc2x0 F.). Heat lost to the environment is precisely balanced by heat produced within the body.
Hypothermia is a condition of abnormally low body temperature. Hypothermia can be clinically defined as a core body temperature of 35xc2x0 C. or less. Hypothermia is sometimes characterized further according to its severity. A body core temperature in the range from 32xc2x0 C. to 35xc2x0 C. is described as xe2x80x9cmildxe2x80x9d hypothermia, 30xc2x0 C. to 32xc2x0 C. is called xe2x80x9cmoderate,xe2x80x9d 24xc2x0 C. to 30xc2x0 C. is described as xe2x80x9csevere,xe2x80x9d and a body temperature less than 24xc2x0 C. constitutes xe2x80x9cprofoundxe2x80x9d hypothermia. Although the above ranges provide a useful basis for discussion, they are not absolutes and definitions vary widely in the medical literature.
Accidental hypothermia results when heat loss to the environment exceeds the body""s ability to produce heat internally. In many cases, thermoregulation and heat production are normal but the patient becomes hypothermic due to overwhelming environmental cold stress. This is a relatively common condition, often resulting from exposure to the elements. Hypothermia may also occur in patients exposed to mild cold stress whose thermoregulatory ability has been lessened due to injury or illness. For example, this type of hypothermia sometimes occurs in patients suffering from trauma or as a complication in patients undergoing surgery.
Hypothermia of either type is a dangerous condition which can have serious medical consequences. In particular, hypothermia interferes with the ability of the heart to pump blood. Hypothermia may be fatal for this reason alone. Additionally, low body temperature seriously interferes with the enzymatic reactions necessary for blood clotting. This sometimes results in bleeding that is very difficult to control, even when normal clotting factor levels are present. These effects and other adverse consequences of hypothermia lead to drastically increased mortality rates both among victims of trauma and in patients undergoing surgery.
Simple methods for treating hypothermia have been known since very early times. Such methods include wrapping the patient in blankets, administering warm fluids by mouth, and immersing the patient in a warm water bath. While these methods are very effective for mild hypothermia, more intrusive methods have been developed for treating severe and profound cases of hypothermia. In particular, methods have been devised to effect direct heating of a patient""s blood. Most commonly, blood is withdrawn from patient""s circulation, passed through external warming equipment, and reinfused back into the patient. Alternatively, the use of heated catheters has been proposed, where a catheter having a heating element near its distal end is inserted into the patient""s vasculature and heat directly transferred into the patient""s circulating blood.
While the direct heating of patient blood can be highly effective, even in treating severe and profound cases of hypothermia, it has been observed by the inventor herein that the excess transfer of heat can cause the patient""s temperature to rise above normal body temperature, resulting in hyperthermia. Hyperthermia can occur, for example, when a hypothermic patient""s metabolism begins to produce substantial amounts of heat at the same time heat is being transferred directly to the blood.
It would therefore be desirable to provide methods for treating hypothermia which further provide for treatment of accidental or incidental hyperthermia. In particular, it would be desirable to develop systems and methods for transferring heat to the blood where heat can be optionally removed if the patient blood or body temperature exceeds a target level. Such methods and devices will preferably employ a catheter for direct heat transfer into circulating blood, but could also be useful with methods where blood is heated externally from the patient""s body. Such systems and methods should further be useful for the treatment of patients who are initially hyperthermic, where the methods and systems provide for initial cooling of the blood and optional heating of the blood should the patient blood or body temperature fall below a target temperature.
The present invention provides apparatus and methods for restoring normal body temperature in patients initially suffering from hypothermia or hyperthermia. The apparatus includes a catheter and a control unit which together permit selective heating and cooling of the patient""s circulating blood. For hypothermic patients, the method will provide for initially heating the blood until a target blood or body temperature has been restored. Heating will be stopped after reaching the target temperature. Even after the heating has been stopped, however, the patient""s blood and/or body temperature will continue to be monitored to assure that the blood or body temperature does not overshoot the target. As discussed above, an initially hypothermic patient can become hyperthermic if the total amount of heat experienced from both patient metabolism and external heating exceeds that necessary to restore normal body temperature. In the case of patients entering hyperthermia, the method of the present invention provides for cooling the patient""s blood, usually using the same intravascular catheter or other apparatus which has been used for heating.
In the case of initially hyperthermic patients, the method of the present invention relies on cooling the patient""s blood in order to reduce the blood and body temperature. Cooling will stop after a target temperature has been reached. The patient""s blood and/or body temperature will continue to be monitored, however, and should the patient enter hypothermia, normal body temperature can then be restored by introducing an appropriate amount of heat to the circulating blood.
According to a first aspect of the present invention, a system for restoring normal body temperature to a patient comprises an intravascular catheter having at least one heat transfer surface, a temperature sensor, and a control unit connectable to the temperature sensor and the catheter. The control unit selectively transfers heat to or from the at least one heat transfer surface in order to achieve a desired target blood or body temperature. The intravascular catheter may comprise a single heat transfer surface for both heat generating and heat absorption, but will usually comprise both a heat-generating surface and a separate heat-absorbing surface. The heat-generating surface will typically comprise a resistance heater, such as a wire coil, and the heat-absorbing surface will typically comprise a metal foil wrapped around the catheter, typically having an exposed area of at least about 2 cm2. In such cases, the control unit may comprise an electrical current source connectable to the resistance heater and a thermal electric cooler connectable to the metal foil. In an alternative construction, the catheter may include at least one flow lumen which permits flow of a heat exchange medium within the catheter past the heat transfer surface. The control unit will then include a heater, a cooler, and a controller for selectively activating the heater or cooler to transfer heat to the heat exchange medium in order to restore normal body temperature to the patient. The heater may be an electrical resistance heater and the cooler may be a thermoelectric cooler.
The temperature sensor will typically be on the catheter and measure the blood temperature. Alternatively or additionally, temperature sensor(s) may be separately attachable to the patient to measure body temperature.
In a second aspect of the present invention, a catheter for restoring normal body temperature to a patient by selectively transferring heat to or from a patient""s blood flow comprises a catheter body having a proximal end and a distal end. The distal end is insertable into a blood vessel, and the heat-generating heat exchange surface and a heat-absorbing heat exchange surface are both disposed near the distal end of the catheter body. Typically, the catheter body will have a length in the range from about 15 cm to 50 cm and a diameter in the range from 1 mm to 5 mm. The heat-generating heat transfer surface will usually comprise an electrical resistance heater, and the catheter will further comprise a connector which connects the resistance heater to an external current source. The heat-absorbing heat transfer surface will typically comprise a metal foil wrapped around the catheter body, and a heat-conductive element will extend through the catheter body to near the proximal end to permit the heat-absorbing foil to be connected to a cooler in a separate control unit. The metal foil heat-absorbing surface will typically have an area of at least 2 cm2, usually being from 4 cm2 to 80 cm2. The heat-conductive element could be either a continuation of the metal foil surface (preferably being insulated in portions which will not lie within the blood circulation), or alternatively could be a metal core composed of a heat-conductive material.
According to the method of the present invention, normal body temperatures are restored to a patient by selectively introducing heat to the patient""s blood flow for hypothermic patients or removing heat from the blood flow for hyperthermic patients. Usually, the heat will be introduced or removed via an intravascular catheter which is connected to an external control unit. Alternatively, the method of the present invention will also comprise the direct extracorporeal heating and cooling of the blood. A temperature characteristic of the patient is monitored, typically being blood temperature and/or body temperature. If the temperature characteristic indicates that initially hypothermic patients have or are about to become hyperthermic, then heat will be removed from the circulating blood to restore normal body temperature. Similarly, if the monitored temperature characteristic indicates that initially hypothermic patients are about to become hyperthermic, then heat will be removed from the blood of those patients until normal body temperature has been restored.
The preferred intravascular catheters will be inserted into a blood vessel, usually being the femoral artery or vein, or the jugular artery or vein. The heat-introducing step comprises introducing heat at a rate between 10 W and 500 W, usually between 50 W and 250 W, while the heat removing step comprises removing heat at a rate from 1 W to 100 W. Preferably, the catheter and system described above will be employed.
For initially hypothermic patients, the temperature characteristic will usually be blood temperature, and the target blood temperature, i.e., temperature at which heating is stopped, will be 36.9xc2x0 C. Should the blood temperature exceed 39xc2x0 C., then cooling will commence. For initially hyperthermic patients, the preferred temperature characteristic will be blood temperature, and the target temperature at which cooling will be stopped will be about 36.9xc2x0 C. Should the blood continue to cool, typically to a temperature of 36xc2x0 C. or below, then blood heating will commence. Is should be appreciated, however, that these temperature targets are nominal objectives, and the methods of the present invention can be practiced with target temperatures which differ somewhat from those just set forth.